1. Field of the Invention
The present invention relates to a method of detecting a base concentration of a photographic film, such as a negative, which has been read out with a film scanner. More particularly, the present invention relates to a method of detecting a base concentration of a film which includes a film image entering process of reading a developed film with an image pickup device to generate pixel values in each of RGB color components. A non-exposure region detecting step is for detecting the edge of each frame-image on the film from the pixel values and specifying as the non-exposure region each line of pixels disposed at the intermediate between any two adjacent frame-image edges from the detected edge data. A base concentration calculating step is for subjecting the pixel values in each non-exposure region specified in the non-exposure region detecting step based on a given arithmetic operation for calculating the base concentration value of the film.
2. Description of the Related Art
It is necessary for printing images recorded on a negative film onto a photographic printing paper, photosensitive material with a higher level of color quality to correct any unwanted artifacts in the images through concerning the intrinsic characteristics of an input system such as a film scanner as well as of an output system such as a photographic printer. For example, a color image data read out from a developed negative film with the use of a film scanner may generally have an undesired effect of bias due to the input characteristics of the color film scanner. Therefore, the color image data has to be subjected to correction processes including color correction data and concentration correction data only when the color scanner has been adjusted initially in the intensity or wavelength of light emitted from its light source or the shutter speed (a charge accumulating duration) of its image pickup device.
The correction processes are preferable to be carried out with reference to the base concentration of each film which may be varied depending on the manufacturer, the degree of sensitivity, and the film type or due to color fading with time.
For the purpose, Japanese Unexamined Patent Publication No. H10-186541 discloses a conventional method of detecting the base concentration of a film which includes, as shown in FIG. 8A, a film image entering process of reading a developed film with an image pickup device to generate the pixel values in each of the RGB color components, a non-exposure region detecting step of detecting the edge, either at right or left, of each frame-image on the film from the pixel values read in the film image entering process and specifying as the non-exposure region each line of pixels spaced by a given distance from the detected edge of the frame, e.g., the center line at the intermediate between any two adjacent frame-image edges, and a base concentration calculating step of determining the mean value of the concentrations of all the pixels in the non-exposure regions on the full length of the film specified in the non-exposure region detecting step as the base concentration value of the film.
However, the conventional method where the base concentration is determined from the mean value of the pixel concentrations in the non-exposure regions at the center line between any two adjacent frame-image edges fails to judge the base concentration at higher accuracy because the line of pixels as the non-exposure region remains not uniform particularly when the distance between frames is too narrow due to the property of a camera such as shown in FIG. 8B or when two adjacent frames are significantly different from each other in the concentration due to over-scene pictures such as shown in FIG. 8C. As a result, such as the color correction will be declined in the uniformity.